2A CUSTOM-TAILORED SUIT ALWAYS FITS BETTER THAN ONE OFF THE RACK.MY...

3.2

A custom-tailored suit always fits better than one off the rack.My uncle, the tailor

The previous section told you how to use predefined functions. This section tells you

how to define your own functions.

■ DEFINING FUNCTIONS THAT RETURN A VALUE

You can define your own functions, either in the same file as the

^main

part of your pro-

gram or in a separate file so that the functions can be used by several different programs.

104 Function Basics

The definition is the same in either case, but for now we will assume that the function

definition will be in the same file as the

main

part of your program. This subsection dis-

cusses only functions that return a value. A later subsection tells you how to define

void

functions.

Display 3.5 contains a sample function definition in a complete program that demon-

strates a call to the function. The function is called

^totalCost

and takes two arguments—

the price for one item and the number of items for a purchase. The function returns

the total cost, including sales tax, for that many items at the specified price. The func-

tion is called in the same way a predefined function is called. The definition of the

function, which the programmer must write, is a bit more complicated.

The description of the function is given in two parts. The first part is called the

function declaration or function prototype. The following is the function declara-function declaration or

tion (function prototype) for the function defined in Display 3.5:

prototypedouble totalCost(int numberParameter, double priceParameter);

The first word in a function declaration specifies the type of the value returned by the

type forvalue returned

function. Thus, for the function

^totalCost

, the type of the value returned is

^double

.

Next, the function declaration tells you the name of the function; in this case,

^total-Cost

. The function declaration tells you (and the compiler) everything you need to

know in order to write and use a call to the function. It tells you how many arguments

the function needs and what type the arguments should be; in this case, the function

totalCost

takes two arguments, the first one of type

^int

and the second one of type

double

. The identifiers

numberParameter

and

priceParameter

are called formal param-

eters, or parameters for short. A formal parameter is used as a kind of blank, or place-formal parameter

holder, to stand in for the argument. When you write a function declaration, you do

not know what the arguments will be, so you use the formal parameters in place of the

arguments. Names of formal parameters can be any valid identifiers. Notice that a

function declaration ends with a semicolon.

Although the function declaration tells you all you need to know to write a function

call, it does not tell you what value will be returned. The value returned is determined

by the function definition. In Display 3.3 the function definition is in lines 24 to 30 of

definition

the program. A function definition describes how the function computes the value it

returns. A function definition consists of a function header followed by a function body .

The

function header is written similar to the function declaration, except that theheader

header does not have a semicolon at the end. The value returned is determined by the

statements in the function body.

The function body follows the function header and completes the function defini-

function body

tion. The function body consists of declarations and executable statements enclosed

within a pair of braces. Thus, the function body is just like the body of the

^main

part of

a program. When the function is called, the argument values are plugged in for the for-

mal parameters, and then the statements in the body are executed. The value returned

by the function is determined when the function executes a

^return statement. (The

details of this “plugging in” will be discussed in Chapter 4.)

Programmer-Defined Functions 105

Display 3.5 A Function Using a Random Number Generator (part 1 of 2)1 #include <iostream>2 using namespace std;3 double totalCost(int numberParameter, double priceParameter);